A car owned by a person on a watchlist of suspected religious radicals has been discovered near Notre Dame cathedral in Paris with seven gas cylinders inside.

The car owner was taken into custody but later released, a judicial official said. A couple, aged 34 and 29, were arrested on Tuesday in southern France in connection with the incident and remain in custody.

The Peugeot 607, which had no registration plates, contained seven gas cylinders, one of them empty on the front passenger seat, two police officials said.

It was found with its hazard lights flashing, as if to attract attention, on Saturday.

Interior Minister Bernard Cazeneuve says he is waiting for a report from investigators on what the possible motives were for the incident.

There was no detonating device present in the car, found on a Seine riverside stretch called the Quai de Montebello, metres from the Notre-Dame cathedral, one of Paris’s many popular tourist attractions.

Documents with writing in Arabic were also found in the car.

More than 200 people have been killed in terror attacks over the past year-and-a-half in France.

France remains on maximum alert after calls by the Islamic State group for followers to attack the country, which is bombing the militant group’s bases in Iraq and Syria.

Florence Berthout, mayor of Paris’s Fifth Arrondissement (District), says the incident highlights the need to beef up security and put more police on patrol in one of the world’s most visited cities.

A state of emergency declared at that time is still in place and gives police extra search and arrest powers but debate still rages over security levels, following a further attack on July 14 in which a man mowed into crowds in the city of Nice, killing 86.

-AAP